542 POPULAR SCIENCE MONTHLY. 



popularly believed on this subject of exhaustion. There is probably no 

 such thing, in a strict interpretation of the term, as an exhausted soil, 

 for it is pretty thoroughly established that although a condition of the 

 soil may have been brought about, as just described, in which it is not 

 suited to the growth of a particular crop, it may be very well suited in- 

 deed to some other crop, and generally is suited to some other crop, and 

 so the terra 'exhaustion^ in this sense is really a relative one only. It has 

 likewise been pretty thoroughly established now that plants, like 

 animals, need a varied, and, as it is sometimes called, 'balanced' ration; 

 tbat there is a particular combination or proportion of various con- 

 stituents in the nutrients which is best adapted to a plant at any 

 particular stage of its growth. For example, it is well understood that 

 for the development of a grain crop, especially at the time when the 

 grain is forming, a certain amount of magnesia is necessary. But 

 if the ratio of the magnesia to the other necessary mineral elements is 

 above a certain figure, the soil solutions become extremely toxic, with 

 the results that the crops fail and the soil is barren. It has been found 

 that the absolute amount of the magnesia which may be present and 

 probably causing the barrenness of a given area is extremely small — so 

 small, indeed, as to be difficult of accurate estimation. But if the ratio 

 of other elements to the magnesia, for instance lime, be raised suffi- 

 ciently high, the plants will do remarkably well, and, up to a certain 

 limit, the more magnesia, the better they will do, so long as the ratio of 

 the magnesia to the lime does not become too high. These views will 

 explain what was meant when it was said above that the fertility or non- 

 fertility of the soil is relative, and it is as a matter of fact, the explana- 

 tion of the earlier remark that the expression 'a poor soil, a poor people, 

 etc.,' may be misleading, if not positively untrue. For it is very prob- 

 able that there is no soil which is cultivatable at all which could not 

 be regarded as a fertile soil for some crop. 



It is possible to modify the conditions as to the fertility in any 

 given soil by tillage, which changes the physical condition of the soil 

 and removes interfering growth from the crop we wish to cultivate; 

 which, on the one hand, promotes capillary rise of water from the lower 

 depths of the soil, or, on the other hand, cuts it off from the surface to 

 prevent its too rapid escape ; and which may improve the physical con- 

 dition of the soil and further promote the natural rate of decomposition 

 or weathering. Again it has been found in modern times that the 

 rotation of crops, or change of crops, will aid the soil. And the reason 

 for this probably lies in large part in the fact, as noted before, that one 

 crop requires a different proportion of constituents in the soil from 

 another, and if it is changed in the proportions of the constituents 

 present, it is still adapted to the succeeding crop, giving time for those 

 which became deficient through the growth of the first crop, to be again 



